The invention relates to an inspection machine for plastic bottles, having a turn-table which is rotatable about a vertical axis and has a plurality of stations to receive the bottles to be inspected, having sensor devices each consisting of a sensor and a mark to be scanned by this and for producing corresponding output signals, and having evaluation electronics for processing the output signals.
The above-mentioned inspection machine is intended primarily for the inspection of returned bottles of plastic material, that is to say of bottles which are returned by the customer and then refilled, which may happen about twenty-five times per bottle. What is in mind are beverage bottles in particular.
It is true that at present the returnable bottle of glass is making a come-back for reasons of protection of the environment but their weight and their fragility are the reason why leading beverage manufacturers are directing their attention to a returnable bottle of lower weight. A bottle of polyethyelene terephthalate or PET is suitable for this. A PET bottle off considerable weight advantages. A full 1.5-liter PET bottle actually weighs no more than its 1-liter counterpart of glass. Thus the housewife carries home 50% more contents with the same physical effort and delivery vehicles pollute the environment less and use less fuel. These are only some of the welcome advantages of PET bottles or of plastic bottles in general.
Before being refilled, plastic bottles must be checked for a number of parameters for which there is no necessity at all in the case of glass bottles. Thus in the case of a plastic bottle, it is necessary to check, before the refilling, whether its bottom, neck and mouth are still at right angles to the bottle axis. Plastic bottles are actually permanently deformable under the action of heat and such deformed bottles cannot then be filled satisfactorily or not without difficulty. For the same reason, the bottle height and the volume of the bottle may vary in the course of time in the case of plastic bottles so that the prescribed amount of filling can no longer be introduced into the bottle.
In addition, however, there are also parameters which have to be checked in plastic bottles as in glass bottles. These parameters include a code which has to be read (for example for sorting purposes) and the pressure-tightness of the bottle. The last-mentioned parameter may be worth checking in glass bottles because of damage to the mouth, through cracks in the wall etc and in the case of plastic bottles there is the additional necessity of checking the pressure-tightness because holes can easily be caused in plastic bottles (for example by pointed objects, glowing cigarettes etc), which in some circumstances may soarcely be visible to the eye.
It is true that inspection devices of the most varied types, even for plastic containers, belong to the prior art, but hitherto no inspection machine has become known which is in a position to carry out all the above-mentioned parameter checks with tolerable expenditure of costs and time.
A method of adjustment and an apparatus for detecting faults in a hollow body of transparent material are known from DE-OS No. 2620046, wherein glass and plastic material are considered as materials and for example cracks, holes, deviations from the specified size, such as inclination of the mouth, ovality and faulty length of the hollow body, as faults to be detected. Obviously, however, these faults can only be detected alternatively in the known fault detection apparatus. The known method and the known apparatus serve more the purpose of simplifying, shortening and making more reliable the adjustment of the evaluation electronics even with a plurality of checking channels, of achieving a partial automation of this adjustment, of taking into account the requirement of greater sensitivity of the fault recognition and of ensuring a supply of operating voltage free of disturbance even with greater sensitivity of fault detection. Even if the known apparatus should comprise a plurality of checking channels, plastic bottles to be refilled could not be checked with it between a bottle washing plant and a filling plant, with a tolerable expenditure of time, because, in the known apparatus, the hollow bodies to be checked have to be brought successively into a checking position, be held and checked in this and only then be conveyed further For reasons of time, this could not be reconciled with the above-mentioned application in which 600 plastic bottles per minute would have to be checked Apart from this, the known apparatus works only with optical checking devices which are likewise unsuitable for the above-mentioned application because bottles which are to be refilled frequently carry with them residues of labels or whole labels from the washing plant, which would make any optical examination with the exception of checking the presence of such foreign bodies, ineffective.
An apparatus for inspecting transparent containers for foreign bodies is known from DE-OS No. 3036502, wherein it is true that the expenditure of time for the inspection is less than in the above-mentioned known apparatus, because the containers are conveyed on a conveyor track past an illuminating device and a scanning device disposed opposite this, but the scanning device works with opto-electronic, light-sensitive elements which leads to the same disadvantages as in the above-mentioned known apparatus. The main purpose of the apparatus for the inspection of transparent containers is therefore also a reliable identification of impurities or foreign bodies such as residues of labels adhering to the container walls. For the said reasons, this known apparatus would, at most be suitable for reading the code but not for checking parameters such as perpendicularity of bottom, neck and mouth, bottle height, volume and soundness.
An apparatus for testing glass bottles is known from U.S. Pat. No. 3,010,310 wherein it is true that the expenditure of time can be reduced because the glass bottles are introduced, by means of a star wheel to which they are fed by a conveyor worm, into a rotating turn-table from which they are removed again, at the end of the test, by a further star wheel, but this known apparatus is adapted only for carrying out one test, namely a bursting test in which the glass bottles are acted upon internally with pneumatic pressure in order to establish whether they meet minimum strength requirements. This known apparatus is neither suitable nor is it provided for the mass checking of plastic bottles for a number of other parameters.
It is the object of the invention to develop an inspection machine of the type mentioned at the beginning so that such different test parameters as perpendicularity of the bottom as well as perpendicularity of neck and mouth of the bottle to the bottle axis, volume and pressure-tightness, bottle height and bottle code can be checked on one and the same machine with tolerable expenditure of time and costs.